pw_rpc: Move system_server backends into targets

Backends for the pw_rpc/system_server facade were defined in the pw_rpc
module, preventing targets from customizing their server configurations
and channels. This change moves the backends into the respective target
directories. Additionally, pw_unit_test's RPC main is updated to use the
system server.

Change-Id: I670e13dd178ab8a7fc8dfa8fade3f82a46d5208c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/28500
Reviewed-by: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Alexei Frolov <frolv@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 020cf7b..9a84842 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -50,6 +50,8 @@
 add_subdirectory(pw_unit_test EXCLUDE_FROM_ALL)
 add_subdirectory(pw_varint EXCLUDE_FROM_ALL)
 
+add_subdirectory(targets/host EXCLUDE_FROM_ALL)
+
 add_subdirectory(third_party/nanopb EXCLUDE_FROM_ALL)
 
 add_custom_target(pw_apps)